Defrosting is very often automatic. That can involve mechanical timers, or software timers, or logic/code of any complexity. All of these things can be badly-implemented and/or become broken or stuck.

Maybe the mode they got stuck on was defrost.

  if n>100 then defrost until y=3

If n is found to be greater than 100 and y subsequently never comes 'round to be 3, then defrost starts and runs forever.